This invention relates to control apparatus of an injection molding machine utilized to mold plastic products.
As is well known, in order to obtain plastic molded products having a uniform quality, it is necessary to change from time to time the operating conditions of the injection molding machine in accordance with various conditions. In a case of 24 hours running, for example, since the ambient temperature greatly varies during night-time and day-time it is necessary to finely adjust the injection speed, injection pressure, etc. in response to such variation of external conditions for preventing production of rejects. Heretofore such fine adjustment has been mad by the experience and feeling of the operator so that it has been impossible to prevent production of rejects.
Various types of the control apparatus have been proposed for preventing non-uniform quality of the plastic products molded by an injection molding machine. In one type, the extent of closure of a metal mold is detected for finely adjusting an injection stroke, while in the other type the resin pressure in the metal mold is detected for effecting a fine adjustment of the injection pressure. In each system, a quality variation caused by the variation in an external disturbing factor is controlled by a predetermined system. For example, an item to be adjusted and a method of calculating a quantity to be adjusted are predetermined. For this reason when the variation of the external disturbance factor is a predetermined one, the fine adjustment is possible with a prior art control method but when the external disturbance factor becomes different the prior art control methods are not effective.
Furthermore, the quality of the molded product is greatly influenced not only by such molding conditions as the injection quantity, the injection speed and the filling stroke but also by the configuration (flat sheet, container, thick wall product, thin wall product, and products having small surface irregularities) and the type of the raw material. Even when the variation in the quality (shrinkage, void, silver streak, etc.) is the same, when the causes resulting in the variation are different, different adjusting means must be used for eliminating the causes. For this reason, a single adjusting means is not always effective for molded products having different configurations and for raw materials having different characteristics.
Furthermore, different control systems have been used for molding products wherein the molding conditions are switched, for example, when the metal mold temperature rises. With this molding system since the new value and the switching time to the new value are predetermined and the molding condition is renewed during a molding cycle, the control system cannot cope with variations in external disturbances and the characteristics of the raw materials so that it has been impossible to mold plastic products having stable quality.